Method for the production of dislocation-free monocrystalline silicon by floating zone melting



March 30, 1965 w. KELLER ETAL 3,175,891

METHOD FOR THE PRODUCTION OF DISLOCATION-FREE MONOCRYSTALLINE SILICON BY FLOATING ZONE MEL'IING Filed Nov. 24. 1961 2 Sheets-Sheet 1 March 1965 w. KELLER ETAL 3,175,891

METHOD FOR THE PRODUCTION OF DISLOGATIONr-FREE MONOCRYSTALLINE SILICON BY FLOATING ZONE MELTING Filed Nov. 24. 1961 2 Sheets-Sheet 2 FIG. 2

United States Patent ice 4 Claims. (61. 23-301 Our invention relates to the production of mono-crystalline rods of hyperpure silicon for electrical purposes. Such rods can be cut into a large number of individual semiconductor discs to be employed in the manufacture of junction-type rectifiers, transistors, and other electronic devices.

It is known to convert polycrystalline semiconductor rods into monocrystal rods by attaching a monocrystalline seed to one end of a rod and passing a melting zone one or several times from the seed crystal to the other end of the rod. In most cases the semiconductor rod is vertically mounted between two holders when this zone-melting operation is being performed. It has become known to keep one of the holders in rotation about the rod axis during the zone-'nelting operation in order to secure a symmetrical growth of the re-solidifying semiconductor material. In most cases, however, monocrystals produced in this manner by crucible-free zone melting exhibit numerous dislocations, twin crystal formations and the like irregularities. These irregularities can be made visible by suitable etching whereby they appear as so-called etch pits on the surface of the ground and etched crystal. Any occurring dislocations are detrimental and, for ex- 35 ample reduce the lifetime of the minority charge carriers in the semiconductor material to an appreciable and undesired extent, when using the semiconductor material having dislocations for the production of electronic devices in accordance with the alloying method, the dislo- 40 cations may cause a non-uniform alloy formation or nonuniform depth of penetration of the alloy being formed. It is, therefore, desirable to avoid the occurrence of dis locations.

According to the copending application of Wolfgang Keller, Serial No, 794,075, filed February 18, 1959, Us. Patent No. 3,159,459, assigned to the assignee of the present invention, the seed crystal used for crucible-free zone melting operations of the above-mentioned type can be given a considerably smaller cross section than the semiconductor rod to which the seed is attached. As a result, the axial temperature gradient from the semiconductor rod to the seed crystal becomes considerably more shallow which in turn results in a reduction of any dislocations and considerable reduction in the possibility that dislocations may grow from the seed crystal into the semiconductor rod.

It has also been proposed to perform the crucible-free zone melting of semiconductor material by passing the melting zone several times in a given direction from the side of the seed crystal toward the other end of the semiconductor rod but effecting a narrowing of the semiconductor rod in the immediate vicinity of the point where the seed crystal is fused to the rod, just prior to performing the 65 last zone-melting pass. This also results in a diminution of dislocations and hence in an increase in lifetime of the minority charge carriers in the semiconductor material; but a completely dislocation-free silicon crystal cannot be achieved in this manner.

It is an object of our invention, therefore, to devise a crucible-free zone-melting method for conversion of poly- 3,175,891 Patented Mar. 39, 1965 crystalline silicon into a monocrystalline rod that affords producing a product substantially free of dislocations.

To this end, and in accordance with our invention, we employ to a certain extent the above-mentioned known zone-me1ting method except for the novel modifications mentioned presently.

We subject a silicon rod, such as any polycrystalline rod, to crucible-free zone melting while the rod is vertically mounted between holders and a melting zone is repeatedly passed from one end of the rod to the other, always commencing at an end where a monocrystalline seed crystal is used together with the rod. For the purposes of our invention, the seed crystal is given a considerably smaller cross section than the rod to be converted, and all zone-melting passes are commenced in the seed crystal. Furthermore, during the last zone-melting pass the traveling speed of the zone in the seed crystal is kept between 7 and mm. per minute, and the silicon cross section at the junction of seed and rod is contracted. by temporarily pulling the rod ends apart at a speed greater than mm. per minute. Furthermore, as the melting zone passes through and beyond the point of contraction, we reduce the traveling speed of the melting zone continuously until it again reaches the full cross section of the silicon rod, and we then continue passing the melting zone through the entire length of the rod at a speed below 7 mm. per minute.

FIG. 1 is a partial sectional and perspective View of apparatus for carrying out the instant process.

FIG. 2 shows a detail of the instant invention.

The housing 2 of the apparatus shown in FIG. 1 generally has the shape of a vertical elongated prism of approximately square cross section. The front wall of the housing is formed by a closure plate 3 which forms the door. The walls of the housing as well as the door are preferably provided with cooling means consisting, for example, of copper tubing (not illustrated) soldered to the walls and the door in order to pass a flow of cooling water through the tubes during operation of the device. An inspection window extends approximately the entire length of the semiconductor rod to be processed and permits observation of the zone-melting operation. A duct 4 for connection to a Vacuum-pump device 5 is mounted on the rear wall of the housing and thence communicates with the interior of the housing through an opening 4a of large diameter and extending over the predominant portion of the horizontal housing width. The opening 4a and the exhaust duct 4 are both located above the bottom surface of the vacuum chamber so that any particles or substances dropping into the bottom of the device cannot enter into the vacuum equipment.

The silicon semiconductor rod 6 is mounted in the processing chamber by means of two holders '7 of which with clamping screws 7b and are mounted in vertical coaxial alignment on respective coaxial shafts 8 and 8awhich pass through respective vacuum-type sealing bushings 8b and 8c of the housing bottom and top to the outside where they are connected with driving and control devices (not shown) which permit displacing the holders longitudinal and/or imparting rotation thereto. The device is further provided with a ring-shaped zone heater consisting of a flat induction coil 11 mounted on terminal blocks 12 of a shifting device 13 which permits movement of the axially narrow heater 11 vertically along rod 6 in order to melt a correspondingly narrow zone of the rod and to displace the molten zone gradually along the rod axis. A semicircular shield 14 of sheet metal protects the device 13 from heat radiation coming from the molten zone and also from deposition 'of evaporated material. Details of the heater and shifting (displacing) device,

which are mounted along one of the corner areas of the prismatic processing chamber, can be found in copending application of Reimer Emeis, Serial No. 830,026, filed July 28, 1959, which application is incorporated herein by reference.

A gripper device, comprising two vertical shafts 18, 18a is mounted along another edge of the prismatic processing chamber, preferably and as shown, near one of the two edges remote from the door. Gripper 15 is fastened by means of set screws to shaft 13 and is longitudinally displaceable so that it can be secured to any desired height. Shaft 18a carries a similar gripper device (not shown) which is likewise longitudinally displaceable. The shafts 18 and 18a carry respective spur gears both shown at 19 which are in mesh with each other and are driven from the outside. The ends of the grippers are both provided with tubular pieces 16 of heat resistant and wear resistant material, which is preferably quartz. The door-like closure plate 3 carries a rubber gasket 28' which is fitted in a swallow-tail groove or otherwise fastened to the door. The door is joined with the housing by double-type hinges 21 whose two hinge axes are approximately located in the plane of the door when the door is closed and tightened. This alfords obtaining a vacuum-tight sealing of the door under the effect of ambient pressure without encountering lateral displacement of the door as a consequence of pressure changes. The door is provided with a handle (not shown) and an observation window 30.

FIG. 2 shows a semiconductor rod 6 in which a molten zone 10 is located. The zone is produced by the heater device, which is, for example, an induction winding 11. 22 is a seed crystal fused to the lower end of the silicon rod 6 and 23 is a melt-down portion as will be further described hereinbelow.

The invention will be further described with reference to the embodiment presented by way of example. The induction winding 11 is energized with high-frequency current for example 4 megacycles per second and is preferably a flat spiral coil as illustrated. This has the advantage that the melting zone can be given a particularly short dimension in the axial direction of the rod, induction winding 4 can be guided along the entire length of the rod wit-h the aid of a transporting or shifting device 13.

The seed crystal 22 which is fused to the lower end of silicon rod 6 has a considerably smaller cross section than the silicon rod. For example, the silicon rod 6 may have a diameter of 12 mm. and a seed-crystal diameter of 3 to The upper end of rod 6 as Well as the lower end of the seed crystal 22 are clamped in respective holders'of which one is shown. These holders can be moved relative to one another in the axial direction of the rod.

It is preferable to employ a hyperpure silicon rod d of such extreme purity that it is practically non-conducting at room temperature. For producing a silicon rod it is therefore necessary to preheat the rod at one end to such an extent that it becomes slightly conducting. The further heating can then be effected with the aid of the heater winding 11 by high-frequency current. The preheating c-an be effected by heat radiation, or by mounting a conducting material, for example a body of molybdenum, at the upper clamping or holding location of the silicon rod 6, which conducting body can then be heated by means of the induction winding 11 and in turn heats the adjacent semiconductor material by heat conductance. The entire method is preferably performed in a highvacuum chamber (as shown) but can also be performed under a protective gas atmosphere.

From the preheated location of the rod, a glowing zone is passed downwardly through the silicon rod 6 until it reaches the fusion point of the seed crystal 22. At this point the silicon rod is subjected to sufiicient heating until the seed crystal 22 becomes molten. It is preferable to give the junction between the thick silicon rod 6 and the thin seed crystal 22 tl1c shape of a cone as shown on the drawing.

After the seed crystal is melted in a narrow zone, the melting zone is passed upwardly through the entire silicon rod up to its upper end. Thereafter a glowing zone is passed downwardly back to the seed crystal, and this method is repeated several times. During this method the travel speed of the heater winding during upward motion of the melting zone may be for example, 3 mm. per minute, Whereas the downward return motion of a glowing but solid zone may occur at a much higher speed, for example of 260 mm. per minute.

When the process has progressed to a point where the entire rod is monocrystalline, the last zone-melting pass is performed in the fol-lowing manner. After the glowing zone, traveling downward, has arrived at the seed crystal, the travel motion of the heater winding is stopped, but the heating by high-frequency current is continued. After a melting zone is produced in the seed crystal, the heater winding 4 is moved upward at a speed greater than 5 per minute, for example 10 mm. per minute. Simultaneously, when the melting zone reaches the location 23 at which the seed crystal 22 merges with the shallow conical portion of the rod 6, the, rod ends are moved apart at a speed of at least 25 mm. per minute until the diameter of the silicon rod at this location is narrowed down to about 2 mm. This can be done by either moving only the upper or only the lower holder away from the other holder, or imparting axial travel motion to both holders. When the silicon diameter is reduced sufficiently at location 23, the relative motion of the rod ends is stopped. If desired, a second constriction can be produced a few millimeters above the first constriction by proceeding in the same manner.

After the rod ends are stopped, the travel speed of the heater winding is continuously reduced. This continuous reduction preferably extends over the length of the conical portion of the rod. When the melting zone reaches the full diameter of the silicon rod, this full diameter being 12 mm. in the example being described hereat, the travel speed of the heater winding has become less than 7 mm. per minute, and is for example 4 mm. per minute. This speed is then kept constant until the melting zone has reached the upper end of the rod. If one of the rod holders, for example the lower one, is placed in rotation during zone melting, such rotation must be free of vibration. Otherwise, such rotation must be discontinued during the last zone-melting pass in order to prevent jarring.

Silicon monocrystals produced by the method described above were found to be completely free of dislocations.

The improvement in crystal quality thus achieved can probably be explained as follows. In the first place, a seed crystal of smaller cross section causes considerably fewer dislocations to grow into the resulting monocrystal than would occur with a thicker seed crystal. The reduction in cross section at the beginning of the last zone-melting pass further contributes to reducing a transfer of dislocations from the crystal into the'main body of the rod. The reduction in dislocations by the diminution of the temperature gradients due to the conjoint action of both just mentioned effects seems to be important. We also believe that any dislocations present in the seed crystal are left behind or, so to say, hung off because of the high traveling speed of the melting zone exceeding the traveling speed of any such dislocations. The generation of new dislocations in the thicker portion of the silicon rod, however, is prevented by the fact that in this portion the traveling speed of the melting zone is greatly reduced. In this connection, the continuous reduction of the rate at which the melting zone travels from the seed to the full cross section of the rod isimportant.

We claim:

1. A process for producing dislocation-free monocrystalline silicon by crucible-free zone meiting which comprises:

(a) vertically mounting a silicon rod by one end;

(b) mounting a monocrystalline seed crystal in vertical alignment with said silicon rod;

(0) attaching said seed crystal to the unattached end of said silicon rod, said seed crystal having a smaller diameter than said silicon rod;

(d) initiating a zone-melting pass in said seed and continuing said zone-melting pass to the opposite end of said attached silicon rod;

(e) repeating step (d) several times:

(1) commencing the last repetition of step (d) in the seed crystal at a rate between 7 and mm. per minute;

(g) as the molten zone of step (7) moves through the junction of said seed crystal and said silicon rod, moving the end of said seed crystal and said silicon rod axially apart at a rate above mm. per minute, to produce a contracted cross section of about 2 mm.;

(h) and reducing the rate of the molten zone pass, as it travels from said contracted cross section of step (g) to the full cross section of said silicon rod, to a value less than 7 mm. per minute.

2. A process for producing dislocation-free monocrystalline silicon by crucible-free zone melting which comprises:

(a) vertically mounting a silicon rod by one end;

(b) mounting a monocrystalline seed crystal in vertical alignment with said silicon rod;

(0) attaching said seed crystal to the unattached end of said silicon rod, said seed crystal having a smaller diameter than said silicon ro-d;

(d) initiating a zone-melting pass in said seed and continuing said zone-melting pass to the opposite end of said attached silicon rod;

(e) repeating step (d) several times;

(1) commencing the last repetition of step (d) in the seed crystal at a rate between 7 and 15 mm. per

minute;

(g) as the molten zone of step (1) moves through the junction of said seed crystal and said silicon rod, moving the end of said seed crystal and said silicon rod axially apart at a rate above 25 mm. per minute, to produce a contracted cross section of about 2 mm;

(12) after the said contracted cross section of step (g) is produced, stopping the axial movement of said silicon rod and seed crystal;

(i) and reducing the rate of the molten zone pass, as it travels from said contracted cross section of (g) to the full cross section of said silicon rod, to a value less than 7 mm. per minute.

3. A process for producing dislocation-free monocrystalline silicon by crucible-free zone melting which comprises:

(a) vertically mounting a silicon rod by one end;

(b) mounting a monocrystalline seed crystal in vertical alignment with said silicon rod;

(0) attaching said seed crystal to the unattached end of said silicon rod, said seed crystal having a smaller diameter than said silicon rod;

(d) initiating a zone-melting pass in said seed and continuing said zone-melting pass to the opposite end of said attached silicon rod;

(e) repeating step (d) several times;

(1) commencing the last repetition of step (d) in the seed crystal at the rate between 7 and 15 mm. per minute;

(g) as the molten zone of step (1) moves through the junction of said seed crystal and said silicon rod, moving the end of said seed crystal and said silicon rod axially apart at a rate above 25 mm. per minute, until a contracted cross section of about 2 mm. is produced;

(it) after the said contracted cross section of step (g) is produced, stopping the axial movement of said silicon rod and seed crystal;

(i) reducing the rate of the molten zone pass, as it travels from said contracted cross section of (g) to the full cross section of said silicon rod, to a value less than 7 mm. per minute;

(j) and maintaining the rate of said molten zone constant at said value less than 7 mm. per minute the remaining length of said silicon rod.

4. A process for producing dislocation-free monocrystalline silicon by crucible-free zone melting which comprises:

(a) vertically mounting a silicon rod by its upper end;

(b) mounting a monocrystalline seed crystal in vertical alignment with said silicon rod;

(0) attaching said seed crystal to the lower end of said silicon rod, said seed crystal having a smaller diameter than said silicon rod;

(d) initiating a zone-melting pass in said seed and continuing said zone-melting pass to the opposite end of said attached silicon rod;

(e) repeating step (:1) several times;

( commencing the last repetition of step (d) in the seed crystal at a rate between 7 and 15 mm. per minute;

(g) as the molten zone of step (1) moves through the junction of said seed crystal and said silicon rod, temporarily moving the end of said seed crystal and said silicon rod axially apart at a rate above 25 mm. per minute, until a contracted cross section of about 2 mm. is produced;

(/1) after the said contracted cross section of step (g) is produced, stopping the axial movement of said silicon rod and seed crystal;

(1') reducing the rate of the molten zone pass, as it travels from said contracted cross section of (g) to the full cross section of said silicon rod, to a value less than 7 mm. per minute;

(j) and maintaining the rate of said molten zone constant at said value less than 7 mm. per minute the remaining length of said silicon rod.

References Cited by the Examiner Zur St-abi'li tat senkrechter Schmelzzoven by Heywang Z. Naturforsch, 11a, pp. 238-43 (1956).

Floating Zone Recrystallization of Silicon, by Keck et NORMAN YUDKOFF, Primary Examiner.

ANTHONY SCIAMANA, Examiner. 

1. A PROCESS FOR PRODUCING DISLOCATION-FREE MONOCRYSTALLINE SILICON BY CRUCIBLE-FREE ZONE MELTING WHICH COMPRISES: (A) VERTICALLY MOUNTING A SILICON ROD BY ONE END; (B) MOUNTING A MONOCRYSTALLINE SEED CRYSTAL IN VERTICAL ALIGNMENT WITH SAID SILICON ROD; (C) ATTACHING SAID SEED CRYSTAL TO THE UNATTACHED END OF SAID SILICON ROD, SAID SEED CRYSTAL HAVING A SMALLER DIAMETER THAN SAID SILICON ROD; (D) INITIATING A ZONE-MELTING PASS IN SAID SEED AND CONTINUING SAID ZONE-MELTING PASS TO THE OPPOSITE END OF SAID ATTACHED SILICON ROD; (E) REPEATING STEP (D) SEVERAL TIMES: (F) COMMENCING THE LAST REPETITION OF STEP (D) IN THE SEED CRYSTAL AT A RATE BETWEEN 7 AND 15 MM. PER MINUTE; (G) AS THE MOLTEN ZONE OF STEP (F) MOVES THROUGH THE JUNCTION OF SAID SEED CRYSTAL AND SAID SILICON ROD, MOVING THE END OF SAID SEED CRYSTAL AND SAID SILICON ROD AXIALLY APART AT A RATE ABOVE 25 MM. PER MINUTE, TO PRODUCE A CONTRACTED CROSS SECTION OF ABOUT 2 MM.; (H) AND REDUCING THE RATE OF THE MOLTEN ZONE PASS, AS IT TRAVELS FROM SAID CONTRACTED CROSS SECTION OF STEP (G) TO THE FULL CROSS SECTION OF SAID SILICON ROD, TO A VALUE LESS THAN 7 MMM. PER MINUTE. 